1. Field of the Invention
The subject matter disclosed generally relates to electrical amplifiers.
2. Background Information
U.S. Pat. No. 8,004,355 discloses various low dissipation, low distortion amplifiers. FIG. 1 shows a representative amplifier circuit from the '355 patent. The amplifier includes a gain/driver amplifier 10 and an output amplifier 12. The amplifier also includes passive impedance networks 12 (Zg), 16 (Z5), 18 (Z4), 20 (Z2), 22 (Z1) and 24 (Z3) that are connected in part to create a feedforward network. The purpose of this feedforward network is to add a correction signal to the output that (ideally) cancels the distortion introduced by the output stage. The feedforward network has an impact on the amplifier drive stage. Not only does the feedforward network carry a distortion correction current but it also carries, by the nature of the circuit that ensures correct distortion cancellation, a significant component of a linear output current. This linear portion may be 100-200 times lower than the main output current but this still represents a significant load upon the driver stage. Not only does the driver stage have to source this linear current hence increasing its quiescent current requirement, the loading by the feedforward network may compromise the stability of the driver stage. Furthermore, the loading of the drive stage increases its inherent distortion.
In the circuit arrangement shown in FIG. 1, Z1 is a resistor R1, Z2 is a capacitor C2, Z3 is a parallel combination of resistor R3 and capacitor C3, and Z4 is a series combination of resistor R4 and capacitor C4. Provided that the bridge is balanced, i.e.
            Z      2              Z      4        =            Z      3              Z      1      then any distortion in the output stage B is eliminated from the output signal VL. This arrangement works well but has two significant limitations: First, the output resistance is Z1//Z3, and given that Z1 is typically a resistor in the range of 0R1 to 0R3, the high frequency output resistance is relatively high (the low frequency closed loop output resistance is reduced by the presence of the feedback loop formed by Z5). Second, the error current that has to be supplied by the output stage of amplifier block 10 is relatively large. Z1 carries most of the output current, causing a significant voltage drop across it. This voltage drop is impressed across Z3, along with any gain loss in stage 12 and the current required to support this voltage drop across Z3 must be supplied by amplifier 10.
The equations detailed above are approximate and are dependent upon the performance of amplifier 10. However, the characteristics of amplifier 10 can at least partially be incorporated into the balance equation and compensated for by appropriate alterations in impedance elements Z1 to Z4. Limitations that can be compensated for by adjustment of the bridge values include gain-bandwidth limitations of amplifier 10 as well as non-zero output impedance.